Magnetic variation anomalies in northern England and southern Scotland

Single-station transfer functions linking the time variations of the vertical and horizontal components of the magnetic field at stations in northern England and southern Scotland have been compiled into a uniform data set. From hypothetical event analysis we show here that there are two distinct anomalies in the Borders region. One runs south-west to north-east, immediately to the south-east of the Southern Uplands Fault; the second follows the Northumberland Basin, and seems to exist because the conductive sedimentary rocks filling the basin create a link between the Irish and North Seas. If the Iapetus suture is marked by a conductivity anomaly, as has been suggested, these results place it beneath the Southern Uplands, unless it is masked by the surface conductor in the Northumberland Basin

Spreading continents kick-started plate tectonics

International audienceStresses acting on cold, thick and negatively buoyant oceanic litho- sphere are thought to be crucial to the initiation of subduction and the operation of plate tectonics, which characterizes the present- day geodynamics of the Earth. Because the Earth’s interior was hotter in the Archaean eon, the oceanic crust may have been thicker, thereby making the oceanic lithosphere more buoyant than at present, and whether subduction and plate tectonics occurred during this time is ambiguous, both in the geological record and in geodynamic models. Here we show that because the oceanic crust was thick and buoyant5, early continents may have produced intra-lithospheric gravitational stresses large enough to drive their gravitational spreading, to initiate subduction at their margins and to trigger episodes of subduction. Our model predicts the co-occurrence of deep to progressively shallower mafic volcanics and arc magmatism within continents in a self-consistent geodynamic framework, explaining the enigmatic multimodal volcanism and tectonic record of Archaean cratons. Moreover, our model predicts a petrological stratification and tectonic structure of the sub-continental lithospheric mantle, two predictions that are consistent with xenolith and seismic studies, respectively, and consistent with the existence of a mid-lithospheric seismic discontinuity. The slow gravitational collapse of early continents could have kick-started transient episodes of plate tectonics until, as the Earth’s interior cooled and oceanic lithosphere became heavier, plate tectonics became self-sustaining

The great glen fault in the Shetland area

WE apologize to Dr Flinn for any disquiet we may have caused by omitting to refer to his recent paper1. We agree with him that the aeromagnetic map2 distinguishes regions of shallow and deep magnetic basement by the smoothness of the contours. We disagree, however, with his use of the general anomaly level as an indication of sediment thickness, which is the distinction between his pattern A anomalies which "probably overlie deep sedimentary basins" and pattern B which "covers sedimentary basins less deep than those of pattern A". The usual geophysical approach is to estimate the depth to the basement using well established quantitative techniques3. One of the main points of our report4 was to present evidence for three deep Mesozoic basins marked C, D and E on our map. These basins are not delineated in Flinn's interpretation, although they do occur within wider tracts which he interprets as sediment covered. © 1970 Nature Publishing Group

Geophysical evidence concerning the structure of the Lewisian of Sutherland, N.W. Scotland

Interpretation of two new gravity profiles across the Ben Stack line and of the I.G.S. aeromagnetic map indicates that the Lewisian biotite-gneisses occurring to the north of this line are underlain at a depth of about 3 km or less by rocks possessing similar density and magnetic properties to the pyroxene-granulites which crop out south of the line. The Ben Stack line forms the fundamental division between these two metamorphic assemblages of the Lewisian and its plane dips steeply towards the south. The gravity profiles also indicate that both the biotite-gneisses and pyroxene-granulites in the vicinity of the Ben Stack line are anomalously low in density. This is attributed to the penetration of both formations by granitic and pegmatitic intrusions of low density. The evidence suggests that the granitic material concentrated in the vicinity of the Ben Stack line occupies too great a volume to be derived solely from the host biotite-gneisses in the immediate vicinity and may therefore possibly be of more extensive origin